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ADL5385ACPZ-R7 データシート(PDF) 18 Page - Analog Devices |
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ADL5385ACPZ-R7 データシート(HTML) 18 Page - Analog Devices |
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18 / 24 page  ADL5385 Data Sheet Rev. D | Page 18 of 24 Using AD9777 Auxiliary DAC for Carrier Feedthrough Nulling The AD9777 features an auxiliary DAC that can be used to inject small currents into the differential outputs for each channel. The auxiliary DAC can produce the small offset currents necessary to implement the nulling described in the Carrier Feedthrough Nulling section. 155 Mbps (STM-1) 128 QAM TRANSMITTER Figure 39 shows how the ADL5385 can be interfaced to the AD9777 DAC (or any Analog Devices dual DAC with an output bias level of 0.5 V) to generate a 155 Mbps 128 QAM carrier at 355 MHz. Because the TxDAC output and the IQ modulator inputs operate at the same bias levels of 0.5 V, a simple dc- coupled connection can be implemented without any active or passive level shifting. The bias level and modulator drive level is set by the 50 Ω ground-referenced resistors and the 100 Ω shunt resistors, respectively (see the DAC Modulator Interfacing section). A baseband filter is placed between the bias and signal swing resistors. This 5-pole Chebychev filter with in-band ripple of 0.1 dB has a corner frequency of 39 MHz. Figure 36 shows a spectral plot of the 128 QAM spectrum at a carrier power of −6.3 dBm. Figure 37 shows how EVM (measured with the analyzer’s internal equalizer both on and off) and SNR, measured at 55 MHz carrier offset (2.5 times the carrier bandwidth) varies with output power. –70 –80 –90 –100 –110 –120 –130 –140 –150 –160 290 420 410 390 400 380 370 360 350 340 330 320 310 300 FREQUENCY (MHz) Figure 36. Spectral Plot of 128 QAM Transmitter at −6.3 dBm Output Power 79 65 –18 0 CARRIER POWER (dBm) 77 75 73 71 69 67 0.7 0 0.6 0.5 0.4 0.3 0.2 0.1 –2 –4 –6 –8 –10 –12 –14 –16 SNR EVM WITHOUT EQUALIZATION EVM WITH EQUALIZATION Figure 37. EVM and SNR vs. Output Power for 128 QAM Transmitter Application CMTS TRANSMITTER APPLICATION Because of its broadband operating range from 30 MHz to 2200 MHz, the ADL5385 can be used in direct-launch cable modem termination systems (CMTS) applications in the 50 MHz to 860 MHz cable band. The same DAC and DAC-to-modulator interface and filtering circuit shown in Figure 39 was used in this application. Figure 38 shows a plot of a 4-carrier 256 QAM spectrum at an output frequency of 485 MHz. –70 –80 –90 –100 –110 –120 –130 –140 –150 –160 –170 430 540 530 520 510 500 490 480 470 460 450 440 FREQUENCY (MHz) Figure 38. Spectrum of 4-Carrier 256 QAM CMTS Signal at 485 MHz ADL5385 1/2 AD9777 I CHANNEL 50Ω 67.5pF 50Ω 67.5pF 156.9pF 156.9pF 124.7pF 124.7pF 50Ω LINE 50Ω LINE 100Ω LINE 0Ω 100Ω LINE 317.4nH 372.5nH 317.4nH 372.5nH 0Ω 200Ω IBBP IBBN 1/2 AD9777 Q CHANNEL 50Ω 67.5pF 50Ω 67.5pF 156.9pF 156.9pF 124.7pF 124.7pF 50Ω LINE 50Ω LINE 100Ω LINE 0Ω 100Ω LINE 317.4nH 372.5nH 317.4nH 372.5nH 0Ω 200Ω QBBP QBBN Figure 39. Recommended DAC-Modulator Interconnect for128 QAM Transmitter |
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